In forming stitches, a lower thread drawing-out device is one of the important mechanisms of the sewing machine, which draws out in advance an amount of the lower thread required to form the stitches, and tightens upper and lower threads crossing each other within the thickness of a fabric under stitching at an upper dead point phase of a thread take-up lever.
In a conventional sewing machine having the horizontal bobbin carrier, the lower thread is drawn out by vertical movement of a lower thread drawing out device 2 provided at a feed dog 100 as seen in FIG. 4.
The foregoing method of drawing out the lower thread depends upon the actuation of the feed dog, and is difficult to obtain desired effects. For incorporating a device to be used to draw out the lower thread into the sewing machine, a very minor space is utilized in relation with a bobbin 4 and stitches 5 as noted in FIG. 1. Therefore, said device could not display sufficient function.
In zigzag stitching, the lower thread is biased to one side of the zigzag width due to relative position between the guiding position of the lower thread from the bobbin and the needle dropping. Causes of said biasing of the lower thread may be summarized as such a case that in the straight stitching the lower thread is, as shown in FIG. 5, supplied from the left side of a bobbin carrier 16 in order to avoid hitch stitching and secure perfect stitch, so that the required amount of the lower thread is changed when the stitches are formed by dropping the needle at the left and at the right in the zigzag stitching as shown in FIG. 6 to FIG. 7. That is, the required amount thereof in the needle dropping at the right side is AL.sub.1. AR.sub.1. C.sub.1 - AL.sub.1. C.sub.1 , while the required amount in the needle dropping at the left side is AR.sub.2. AL.sub.2. C.sub.2 - AR.sub.2. C.sub.2 . Thus, the required amount of the lower thread in the needle dropping at the right side is larger than at the left side.
For avoiding said biasing, the lower thread 6 is passed through a center of the bobbin carrier 16, however in this case when the needle drops at the left side, the hitch stitching is caused.
In view of these circumstances, the straight stitching and the zigzag stitching require different lower thread guiding positions respectively. However it has been conventionally difficult to incorporate the lower thread guiding device in the narrow spaces as said above.